1. Field of the Invention
The invention is related to stopper rod tapping assemblies, and, in particular, to a stopper rod tapping assembly used in casting filaments from molten metal sources.
2. Description of the Prior Art
Bottom pouring or tapping of molten metal from crucibles is well-known in the steel industry for eliminating some of the disadvantages related to top pouring. As used herein, the term "metal" includes metal alloys. A variety of bottom tapping assemblies have been utilized; one such assembly is the well-known stopper rod tapping assembly. In this technique, a vertical standing rod is seated against a tapping orifice in the bottom of the crucible while the metal in the crucible is brought to a temperature sufficient for casting a molten stream. At that time, the stopper rod is lifted, and the molten metal exits from the crucible through the tapping orifice. Often, it is convenient in processing certain metal alloys to seal the crucible in some fashion and to provide a protective atmosphere over the surface of the melt, thereby preventing any adverse chemical reactions, such as with oxygen, from occurring. This technique is often used, for example, in casting amorphous metal alloy filaments directly from the melt. Typically, these amorphous metal alloys contain about 75 to 85 atom percent transition metals and about 15 to 25 atom percent metalloids; many of these elements react with oxygen. It is often convenient in sealing the crucible to provide both a gas inlet and a gas outlet to enable inert gas, such as argon, to be introduced under flowing conditions above the surface of the melt. Closing the gas outlet and lifting the stopper rod substantially simultaneously enables an over-pressure of inert gas to be built up over the surface of the melt in order to expel the melt through the tapping orifice at a controlled rate.
In general, when the tapping or jetting of molten metal through a nozzle is intentionally interrupted, as by the temporary closing of a valve, residues of the molten metal tend to remain in the nozzle orifice. Such molten metal residues may then become oxidized by the ambient atmosphere to which it is exposed and form solid oxide residues which then interfere with or prevent subsequent jetting of the molten metal when the valve is reopened. In the case of bottom tapping where the stopper rod serves as the valve, this buildup of residues poses a difficult problem, especially where it is desired to control the rate of extrusion, as in the casting of metal alloy filaments.